Energy, power and resistance
Potential difference measures energy transferred per unit charge. Combine it with current to calculate power, and investigate how components respond to changing voltage.
Specification outcomes (19)
- 4.2.1(a) Recognise and use standard circuit symbols.
- 4.2.1(b) Construct and interpret circuit diagrams using correct symbols.
- 4.2.2(a) Define potential difference (p.d.) and state the unit volt.
- 4.2.2(b) Define electromotive force (e.m.f.) of a source such as a cell or power supply.
- 4.2.2(c) Distinguish between e.m.f. and p.d. in terms of energy transfer.
- 4.2.2(d) Use energy transfer equations W = VQ and W = EQ.
- 4.2.2(e) Use energy transfer for charged particles: eV = ½mv² for electrons and other charged particles.
- 4.2.3(a) Define resistance using R = V / I and state the unit ohm.
- 4.2.3(b) State and apply Ohm’s law.
- 4.2.3(c)(i) Describe and sketch I–V characteristics for resistor, filament lamp, thermistor, diode and LED.
- 4.2.3(c)(ii) Use techniques and procedures to investigate electrical characteristics of ohmic and non-ohmic components.
- 4.2.3(d) Describe how the resistance of a light-dependent resistor (LDR) varies with light intensity.
- 4.2.4(a)(i) Define resistivity using R = ρL / A.
- 4.2.4(a)(ii) Use techniques and procedures to determine the resistivity of a metal.
- 4.2.4(b) Describe variation of resistivity of metals and semiconductors with temperature.
- 4.2.4(c) Understand the negative temperature coefficient (NTC) thermistor and variation of resistance with temperature.
- 4.2.5(a) Use and apply power equations P = IV, P = I²R, and P = V² / R.
- 4.2.5(b) Use energy transfer equation W = VIt.
- 4.2.5(c) Recognise the kilowatt-hour (kWh) as a unit of energy and calculate cost of electrical energy.
H156 v2.0 · May 2026
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A wearable warning indicator is connected to a supply that maintains 3.0 V across the indicator whenever it is on. During each 0.20 s pulse, a charge counter measures 0.012 C passing through it. Current is constant during a pulse and zero between pulses. The indicator produces one identical pulse every 1.0 s; it produces 600 pulses in 10 minutes. Ignore any current used by other electronics. (a) Calculate the current while the indicator is on. [2] (b) Calculate its electrical power while it is on. [2] (c) Calculate the electrical energy transferred during one pulse using charge and potential difference. [2] (d) Calculate the total electrical energy transferred in 10 minutes and the average power over that time. [3] (e) A pupil says the current leaving the indicator must be smaller because the indicator has used up some electrons. Explain why this statement is wrong, distinguishing charge transfer from energy transfer. [2]
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